1. Field of the Invention
The present invention relates generally to electrical devices, and particularly to ground fault circuit interrupter (GFCI) devices.
2. Technical Background
By way of background, power is provided to electrical appliances through a pair of electrical conductors connected to a power source provided by the local power utility. One conductor is a “hot” wire and the other conductor is a neutral conductor. The hot conductor is also commonly referred to as the line. The line and the neutral provide the load (e.g. the appliance) with 120 volts of alternating current (VAC) or 240VAC. Ground faults occur when there is an imbalance between the current flowing in the line and the neutral, or if the neutral becomes grounded at the load.
A GFCI is a safety device that, if used properly, may help prevent electrocution because of ground faults. Ground fault conditions pose a significant threat to safety and may result in serious injury or death. The GFCI is configured to automatically detect the fault condition and open the circuit to eliminate the fault condition. GFCIs can typically detect the presence of a ground fault as small as a few milliamps and open the circuit within a fraction of a second to eliminate the dangerous fault condition.
In a conventional GFCI, current differentials between the line and neutral are sensed by a current differential sensing transformer. Grounded neutral conditions are sensed by a second ground neutral transformer. Both of the transformers are coupled to a fault detection circuit. Upon detecting a ground fault condition, the fault detection circuit directs a solenoid to actuate a circuit breaker. The circuit breaker, in turn, opens the circuit to eliminate the dangerous condition. Unfortunately, the above described conventional GFCI has limited functionality that is unable to cope with certain problems.
One such problem includes the possibility of an installer mis-wiring the line/load in the field. A variety of methods are used to prevent or attempt to prevent mis-wiring with varying levels of success. Labels and installation instruction sheets have been used to prevent mis-wiring. Of course, instructions can always be ignored by the installer. Another potential problem includes the possibility of solenoid failure. Obviously, if the solenoid fails, or if the circuit driving the solenoid fails, the GFCI will be inoperable, and hazardous ground fault conditions will go undetected. Preventing the problems associated with a defective solenoid driving device is inherently more problematic.
GFCIs have been provided with mis-wiring protection circuits, and built-in means for detecting defective internal GFCI components, such as a defective solenoid. However, these circuits have proved to be problematic during some test procedures. First, mis-wiring protection circuits often produce differential currents that skew test results. Also, some test procedures may-cause the mis-wiring protection circuit to burn out, making the device unsuitable for sale. What further exacerbates the problem is that certain standards, such as Underwriters Laboratories Standard 943 (UL 943), do not allow the manufacturer to open the GFCI device after the device has been tested. Thus, if a circuit does fail during testing, the device must be scrapped.
Therefore, it is desirable to provide a GFCI device that is amenable to rigorous testing, such as the test procedures provided by UL 943.